Telephone line supervision is typically accomplished by applying battery voltage to the line via feed resistors and comparing the voltage on the ring conductor to a predetermined reference level. A common problem which line supervision circuits must deal with is the presence of longitudinal AC voltages induced by nearby electrical power lines. A well-known approach to the problem involves filtering out these longitudinal voltages using an analog, resistor-capacitor filter network. However, the large size of the capacitor required to satisfactorily filter the power frequency imposes definite limits on the size and cost reductions that can be achieved for supervision circuits that rely on this approach. A second known approach, which deals with AC longitudinal rejection by measuring the differential voltage between the tip and ring conductors, is not applicable to ground start lines since only tip supervision is used for those lines. The present invention is a voltage detector circuit advantageously employing a digital technique to detect changes in DC voltage in the presence of a substantially fixed frequency AC voltage. The circuit of the invention can be readily implemented using integrated circuit technology to achieve substantial size and cost reductions over known voltage detectors.